Conformal coatings have been used for many years in the electronics industry to protect electrical assemblies from environmental exposure during operation. A conformal coating is a thin, flexible layer of protective lacquer that conforms to the contours of an electrical assembly, such as a printed circuit board, and its components.
There are 5 main classes of conformal coatings, according to the IPC definitions: AR (acrylic), ER (epoxy), SR (silicones), UR (urethanes) and XY (paraxylylene). Of these 5 types, paraxylylene (or parylene) is generally accepted to offer the best chemical, electrical and physical protection. However, the deposition process is time consuming and expensive, and the starting material is expensive.
Parylene is polymer with the following structure:
Parylene is deposited using a three stage vapour deposition process. A solid precursor is heated under vacuum and sublimes. It is important to appreciate that parylene, although sometimes erroneously called “paraxylene”, is not in fact prepared from the compound paraxylene. In fact, the precursor is [2.2]paracyclophane:

The chemical vapour is then passed through a high temperature furnace at around 680° C., so that the precursor splits into a reactive monomer. This reactive monomer then feeds into a deposition chamber and polymerizes on the surface of the substrate. Typical coating thicknesses for parylene are between 5 and 25 microns.
The parylene deposition technique described above is not ideal because of the high cost of the starting material, the high thermal energy consumption during monomer generation, the high vacuum requirements and the low growth rate.
There is therefore a need for conformal coatings that offer at least similar levels of chemical, electrical and physical protection as parylene, but that can be manufactured more easily and cheaply. For certain applications, it can also be important that the conformal coatings can withstand the conditions used in rework processes and reflow processes.